Transmission x-ray imaging is widely used for the detection of prohibited items within air and sea cargo containers, vehicles and railcars. However, while x-rays are extremely useful in identifying certain hidden objects, they are not very useful in identifying a wide variety of other potentially extremely dangerous items such as drugs, explosives and special nuclear materials (SNM).
Transmission x-ray images become even less practical to use in open environments such as warehouses or the hold of cargo ships, etc. Furthermore, considering that a particular prohibited material may be in a magnetic steel container as are commonly used in the shipping industry, the use of alternative techniques involving electromagnetic fields or radiation with the possible exception of hard x-ray or gamma radiation is practically precluded.
An additional problem in detecting such prohibited materials arises out of the total lack of knowledge as to whether such materials are indeed present in a particular shipment which may well involve a plurality of shipping containers or other packages stored or arriving in storage area, a warehouse or present on the deck or cargo hold of a ship.
Some fissile materials emit copious amounts of gamma rays from their predominant radioactive alpha decays, which are easily detected in gram quantities of material in seconds if the material is close and unshielded. However, the energy of the intrinsic gamma rays for fissile material is rather low and easily absorbed. Therefore, with even modest amounts of shielding, the gamma-ray signatures of even kilograms of fissile material disappear into the background, rendering their detection through characteristic gamma rays difficult if not impossible in limited amounts of time.
All fissile material has a finite probability of undergoing spontaneous fission instead of alpha-decaying, thereby emitting neutrons that can be detected. Thus, the type of prohibited materials of most interest are themselves generally neutron emitters and also have very specific neutronic properties, making neutron detection and imaging an ideal method for detecting and identifying such materials. Neutrons are much more difficult to absorb and therefore shielding such prohibited material becomes more difficult, Therefore, neutron detection offers a better opportunity to detect shielded fissile material.
Additionally many non-neutron emitting materials can be identified by their specific responses to irradiation with neutron radiation. U.S. Pat. No. 5,838,759 issued Nov. 17, 1998 to R. A. Armistead, discusses the possibility of using such a neutron method for cargo containers but concludes that such systems are impractical.
U.S. Pat. No. 5,278,418 issued Jan. 11 1994 discloses a method for detecting a predetermined amount of oxygen and nitrogen in a luggage type container. The disclosed system is confined to detection of suspect materials in small containers whose locality is well known, such as a luggage conveyor belt. The disclosed method would be practically impossible to implement in an open environment as that of a warehouse containing a plurality of shipping containers.
In Jan. 2001, Brookhaven National Laboratory disclosed a high precision, high efficiency thermal neutron detector using multiwire proportional chambers filled with 3He. Neutrons enter the detector chamber through an aluminum window and collide with the 3He generating protons and tritons which produce ionization electrons that drift through an upper wire cathode producing an avalanche on the nearest anode wire or wires.
The upper cathode and anode wires run in the same direction. There is also a lower cathode formed of metal strips running at right angles to the anode wires. The avalanche induces positive charge on both the upper and lower cathodes. The sampling of induced charge with cathode wires or strips yields the center of gravity of the anode avalanche with high precision, providing a two dimensional position indication of the collision locus of the neutron with the 3He.
While all of the above systems will detect the presence of neutron radiation when placed in the vicinity of a neutron radiation source, none will provide information regarding the direction of the source relative to the detector or an image of the source distribution and location as observed from the position of the detector.
There is therefore still a need for a neutron radiation detector imaging system that, when placed in the general vicinity of a source or a plurality of sources of neutron radiation, will provide an image of the location and or distribution of such source or sources, thereby permitting easy identification of a suspect container with a reasonably high degree of certainty.